Audio amplifier circuit



Feb; 13, 19 10. L. E.- ORNE AUDI-0 AM1?LIFIER cIRcui'r Filed April 1, 1938 I000 Frequency Patented Feb. 13, 1940 v "use? PATENT critics I AUDIO AMPLIFIER CIRCUIT Laurence E. Orne, Melrose, Mass. Application April 1, 1938, Serial No. 199,473

4 Claims.

This invention relates to electrical thermionic amplifiers and especially to such amplifiers as operate the so-called audio frequencies, namely frequencies fromto 20,000 cycles per second.

Amplifiers of this type are intended to give high fidelity or high quality amplification of speech, music or other sound effects.

The primary object of my invention is to provide an amplifier which will amplify with equal. amplitude signals of all frequencies within the range of audibility. Another object of my invention is to accomplish this amplification with as simple a circuit as possible consistent with l the desired degree of fidelity of reproduction,

The natural tendency of thermionic amplifiers of ordinary construction is to amplify disproportionately certain frequencies with respect to certain other frequencies, with the result that sound output is not a faithful reproduction of the sound input. In many cases, it is the higher frequencies that are amplified disproportionately. Inorder-to compensate. for this disproportionate amplification of the higher range of frequencies, it has been found possible through suitable circuits to feed back in various ways a portion of the output ofthe last stage of amplification or of some intermediate stage to the grid circuit of the first amplifying tube.

This return voltage is applied to the grid circuit of the first tube insuch a way that it will modify the output of the first tube and consequently the output of the succeeding stage at the higher frequencies without decreasing the output of the lower frequencies. 'In this .way

the amplification of all frequencies may be held substantially proportionate so that faithful reproduction is obtained.

My invention is particularly concerned, there- I fore, withiproviding a circuit in which the means of feed back provides such control of the grid circuit voltage of the first tube that the output of the amplifier will give a flat gain. curve.v

Previous methods of accomplishing this. result have'taken several forms as, for instance, the

use of an odd number of stages greater than one,

thus returning to the grid of the first tube a voltage out of phase with the grid voltage of this tube. Another method of prior art ,is to use push pull amplification. in the second stage andthus obtain the necessary phase reversal with .twostages, but this method involves the use of three tubes or an ordinary vacuum tube and a tube with two-sets of elements therein.

By my invention, however, it is contemplated to compensate for the variations in amplificarny' amplifier circuit includes two pentode vactions which would naturally be obtained at the higher and lower frequencies by returning a por tion of the output current of the second tube tube.

By my circuit a current flows through the control grid circuit network of the first tube in such a manner as to oppose the voltage produced in this'circuit by the secondary coil ofthe input transformer. By suitable selection of thewari ous'electrical constants of the circuit, it is possible to secure a degenerative action in the circuit which varies automatically, with the fre-' quency andprevents any tendency for the gain of the amplifierto change regardless of the frequency. As stated above, this circuit. is designed to hold the gain curve flat throughout the entire range of audible frequency as this gives the most desirable reproduction.

With the constants that 'I have determined as I being suitable, a fiat gain curve is accomplished, but I have also found that by varying certain of the constants, it is possible to modify the response of the circuit through certain particular bands of frequency, if such modification is 'desired. v

Fig. 1 of the drawing shows the circuit.

Fig. 2 gives graphically the results of the inby the amplifier. H t

Referring now to. Fig.1, it will be seen that uum tubes connected "in cascade by means of a 5 resistance coupling'with the addition to the conventional'form shown of the feed back circuit comprising the resistance R1 and the condenser C1 in series, which circuit provides a path from the plate of the second tube. to the cathode and suppressor grid of the first tube.

Let us consider the action of the circuit shown in Fig. 1 without including, for the moment, the feed fack circuit through R1 and C1. The potential of point 'G when a constant current is flowing in the control grid circuit of the first tube in the direction of the arrow is lower thanthe potential at point K. When no current flows in the primary winding of the input transformer, the potential of the grid is the same as the potential of the point K. Let us assume a signaltential is minus 3 volts, a signal might increase it to minus 1 volt. There will be a resulting increase in the audio frequency plate current of the tube which change will be amplified by the second tube to increase still further the audio frequency plate current in the second tube. This increased current results in an increased output current from the second'tube, and for any given 20 log-10% In terms of the ratio of voltage amplification the gain in decibels may be expressed as E 20 log. wig? This statement holds when the load resistance equals the input resistance.

Without some means of feed back the ordinary amplifier will show a varying gain with varying frequency because the space charge current in the plate circuit flows through that portion of the grid circuit which is connected to the cathode. condensers in the grid return circuit offer a higher impedance path to low frequency currents than they do to high frequency currents, andat higher frequencies the losses in the transformers increase with frequency.

This impedance effect results in an increasing degenerative action as the frequency becomes lower because it reduces the effective change in potential in the grid circuit which causes the operation of the tube, while at higher frequencies the degenerative action is less, but above a certain limit the losses in the transformers increase and the gain decreases.

In the circuit constituting my invention, the constants are so selected that the desired gain will be obtained at substantially the lower limit of audible frequency andthe values for the feed back path through C1 andRi are so selected that they will cause sufficient current to flow through them which may be introduced into the grid return network, and as a result of this additional current flowing into the grid network, a voltage will be set up which will oppose in varying degree the input voltage and tend to hold the entire amplifier stable.

Briefly, then, the feed back effect is lowest at low frequencies'when the degenerative effect of the low frequency currents in the plate return circuit is'highest, and increases as the by-passing condensers handle more and more of the plate return current. At the higher frequencies, the feed back path offers less impedance to the path of the fed back current, so the degenerative action of the feed back circuit is greater.

The feed back effect is limited more and more at the higher frequencies,however, by the value of the resistance R1 so that the feed back curve flattens out as the frequency increases. By

proper selection of Cl and R1 the desired flatness.

At lower frequencies, the by-passing varying proportions of the plate current can be made to return through the network to vary the degenerative effect according to any desired curve.

The results obtained with an amplifier connected as described herein have been attainable formerly only with three or more tubes or with much more apparatus than is required in the present invention.

A particular form of the amplifier as constructed and found to produce the results claimed for the invention is described as follows:

The input, which may be derived from any audio frequency source which it is desired to amplify, is connected to' the primary. 2 of the input transformer, which for best results should be shielded,

One terminal of the secondary 4 of the input transformer is connected to the control grid of the first stage vacuum tube 5 which may be a pentode tube such as the RCA #57 tubeor any similar tube. The other terminal of the secondary l is connected to the grid return network at a point between the ance 8.

The suppressor grid and cathode of the first tube 0 are connected together and connected to a resistancev l9 for which a suitable value may be 800 ohms, which in turn isyconne'cted into the first stage network consisting of condensers 6 and i2 and resistances 8 and Hi. The constants arid function of the network will be described la er.

The filaments of both the first stage tube 6 and the second stage tube 16 serve merely as heaters for the cathodes and may be supplied from any-suitable power source. For convenience the filament power supply is not shown on the diagram.

The plate supply of both tubes consists of a source of direct current liiwhich may be abattery or any source of direct current without 'excessive ripple voltage. The potential of' the direct current source should be approximately 220 volts. A drop wire, bleeder resistor, or potentiometer 20 is connected across the power supply and a condenser 6 and resist-' 25 tap 2.2 taken 01f to supply the proper voltage v for the screen grid of tube 6. For the Type 57 tube this voltage should be about 30 volts.

Direct current source 58- also supplies plate voltage for both tubes and is filtered by'means of the choke coils 24 and 2 $*and condensers 28,-

The plate load of the first tube consists of the resistance 35 a suitable valuefor which is about 125,000 ohms. Condenser 36 of fairly large capacity, about 4 mf., provides a by-pass for the audiofrequency component of plate current around the D. 0. power supply circuit, returning'it to the cathode through the resistance l0.

The second stage vacuum tube It may be a pentode tube such as the. RCA type 59 or any tube having similar characteristics. The second stage is connected to the first stage throughthe condenser 38 which has a'value of about 0.4 mf. and is connected between the plate of the first tube and the control grid of the second tube. Resistance M] is connected from the control grid of the second tube to the grid return network resistance 44.

The suppressor gridand cathode of the second tube are connected together and connected to the grid network consisting of condensers 42 and 46.

' the plate voltage direct current source I6 through Condenser 56, a suitable value forand resistances 44 and 48. This network and its function willbe described later. Y

The plate of the second tube is connected to one terminal of the output transformer primary coil 50, whose secondary 52 may connect to the loud speaker or other translating device. For best results a shielded transformer should be used, to prevent cross-talk and undesired feedback.

primary 50 connects to the positive terminal of choke 24. which is 2 mt. is connected betw'een'the cathode and the terminal of the output transformer primary 50 not connected to the second tube plate, and furnishes a by-pass for the audio frequency component of the second tube plate current around the D. C. power supply.

The screen grid of the second tube is connected to the same terminal of the output transformer primary as condenser 56 in order to maintain it at a high positive potential with respect to the cathode. I

The plate of the second tube I6 is connected to the cathode of the first tube 6 through condenser 58 or C1 and resistance 60 or R1 of values about 0.25 mi. and 250,000 ohms respectively.

v The negative side of the D. C. high voltagev supply, the metal mounting base, all metal equipment covers, and the common connections of'resistanceB, M, 44, and 48 are connected to the ground. 1 The grid return networks are connected as follows: h 4

In the first stage, the cathode'and. suppressor grid are connected together and connect to res sistance II], for which a suitable value is about 800 ohms; the other terminal of resistance l0 connects to an impedance consisting of resistance M and condenser l2 in parallel, the other end of which is'grounded; and to an impedance consisting of condenser 6 and resistance 8 in series the other end of resistance 8 being grounded. As

previously stated, the grid circuit return from input transformer secondary connects to the common terminals of condenser 6 and resistance 8. The by-passing condenser 36 instead of being connected directly to the cathode as is usually donein. prior practice, connects to the cathode through resistance H), which is thus common to the grid circuit andthe plate circuit. In the second stage, the network consisting of condensers 42 and 46 and resistances 44 and 48 is similar to the first stage network except that the 800 ohm resistor I0 between the cathode and network proper is omitted. The second tube, cathode is connected directly to the network, and the plate lay-passing condenser 58 is connected directly to the cathode.

Suitable values for use in. the networks have.

' been found to be as follows:

Condensers Resistances Ohms 6-1 microfarads. .4 l0800 128 microfarads. 8-60000 42-l microfarads. 14-2500 46-16 microiarads 44-10000 48-400 The amplifier, circuit operates in the following manner: 7

Alternating current of varying frequency and voltage flows in the primary, 2 of the input trans- The other terminal of the output transformer fact that more cathode-plate former inducing a voltage of similar Wave form in the secondary 4.

This voltage induced in the secondary, together with the difference in potential existing in the firststage grid network because of the cathode current flowing through the network, is impressed on the control grid of the first tube, causing a variation in the plate current of the first tubein accordance with the well-known action of the vacuum tube. This plate current flows through the resistance 34 and the voltage across 34, within the limits between which the tube has a linear characteristic, is a definite multiple of voltage impressed on the grid.

The current flowing through resistance 34 and w from plate to cathode in the first tube 6 can re-" turn from the cathode through either of two paths; currents of the higher frequencies will flow in most part through condenser 36 which ofiers a low impedance to high frequencies; currents of the lower frequencies, however, tend to I return. through the grid return network,.- especially resistance M. The lower frequency currents thus tend to set up an E. M. F. in the grid returncircuit opposing the E. M. F. induced by the input and causing a degenerative action,

which is most pronounced at lower frequencies.

The output of the first tube 6 is transmitted to the second, tube 16 by means of the condenser 38' which blocks direct current but impresses the alternating component of the voltage across. resistanceiit on the control grid of the second, w

Resistance 40 furnishes a grid leak for the control grid of the second tube.

tube.

: In accordance with the operation of the second vacuum tube It the plate current varies with the grid voltage to produce a varying current inputput'transformer primary 50 which should be 'a a degenerative effect in the second stage similar to that which occurs in the first stage, and the result of this degenerative effect is to produce definite multiple of the input. There is, however,

a gain curve which tends to vary with frequency.

This degenerative effect is compensated by the feedback.

In accordance with the well-known vacuum tube theory, each single stageot amplification shiftsthe phase consequently the output of the second tube is in phase with the input. A, portion of the alternating component of the plate current of the second tube flows through the condenser 50 and resistance 60 to the cathode of the first tube. This current flows through resistance l0 in a direction tending to make the grid of the first tube more negative, hence tending'to decrease the output of the first tube and consequently the output of the second tube.

However, as the output of the second'tube'decreases, the feed back current also decreases. Hence it can be seen that the feed back current tends to stabilize the amplification at any given frequency.

The condenser 50 is selected with a rather small capacitance, about 0.25 mi, so that the feed back intend to limit myself thereby but only by the very. little impedance to the feed back current the resistance 60 limits the upper valueof the feed back current so that its degenerative efiect will not cause the gain curve to drop oii at fre-" i quencies of 10,000 or thereabouts and above.

Having described my circuit and having mentioned the values that have been found suitable to produce a satisfactory result, I wish, however, it would be distinctly understood that I do not appended claims.

I claim:

1. A vacuum tube amplifier circuit comprising two stages of pentode vacuum tubes coupled in in. cascade with a feed back connection comprising a resistance and a capacitance connected in series between the plate of the second tube and the cathode of the first tube for the purpose of feedingback a portion of the output of the second tube to the first tube, networks comprising interconnected resistances and capacitances connecting the control grids with their corresponding cathodes, the network of the first tube being connected between the cathode of the first tube and .5 the controlgrid of the first tube, and comprising a resistance one terminal of which is connected to; the cathode and suppressor grid circuit of the said first tube, and the other terminal of said resistance connected to (a) one terminal of a corn .denser, the other terminal of which connects both (2)) one terminal of a path comprising a resist 'to one terminal of the secondary winding of the input transformer and to one terminal of a s c+ 0nd resistance, the other terminal of said second resistance being connected to a common ground,

ance and a capacitance in parallel, the other terminal of said parallel resistance and capacitance being connected to the common ground, (a) one terminal of a capacitance comprising one or more relatively large condensers in parallel,

the other terminal of said condensers being connected to one terminal of the resistance forming the plate return circuit of the first tube.

2 A vacuum tube amplifier circuit comprising two stages of pentode vacuum tubes coupled in cascade with a reed back connection compr sing a resistance and a capacitance connected in series between the plate or the second tube and the cathode of the first tube for the purpose of feed.-

ing

""-' tube to the first tube, ne w s Comprising inter- 55 the control grid of the first tube and comprising connected resistances and capacitances connecting the control grids with their corresponding cathodes, the network of the'first tube being connected between the cathode of the first tube and a resistance, one terminal of which is connected to the cathode and suppressor grid of the said first tube and the other terminal of said resistance connected (a) to the terminal of a. con-- denser, the other terminal of which connects both to one terminal oi the secondary winding of the input transformer and to one terminal of a second resistance, the other terminal of said second resistance being connected to a common ground, (19) one terminal of a path comprising a resistance and a capacitance in parallel, the other terminal of said parallel resistance and capacitance being connected to the common ground, (c) one terminal of a capacitance comprising one or more relatively large condensers in parallel, the other terminal of said condensers being connected to one terminal of the resistance forming the plate load of the first tube, the netback a portion of the output of the second" work of the second tube being connected between the cathode of the second tube and the control grid of the second tube, said secondnetwork com prising a resistance andcapacitancein parallel, one terminal of which is connected to the common nectedto (a) the cathode and suppressor grid of the first tube, (b) one terminal of-a' condenser,

the other terminal of said condenser being connected to the screen grid of the second tube, (0) the terminal of another condenser, the other terminal of which is connected both to a path including a resistance to the control grid of the second tube and to a path tothe common ground including another resistance,

cascade with a feed-back connection'comprising a resistance and a capacitance connected in series between the plate of the second tube and the .15)? 3. A vacuum tube amplifier circuit comprising two stages of pentode vacuum tubes coupled in cathode of the first tube for the purpose of feed- 3 a ing back a portion of the output of the second tube to the first tube, networks comprising interconnected resistances and capacitances connectthe control grids with their corresponding cathodes, the network of the second tube being connected between the cathode of the second tube and the control grid of the second tube, said net'- work comprising a resistance and a capacitance in parallel, one terminal of which is connected to the commonground and the other terminal of whicbfis connected to (a) the cathode and suppressor grid of the second tube, (17*) one terminal of a condenser, the other terminal of said con-- denser being connected to the screen grid and plate return circuit of the second tube, (0) one terminal of another condenser, the other terminal "of which is connected both to a path includ I ins a resistanceto thecontrol grid of the second tube and to a path to the common ground includ me another resistance.

i. A vacuum'tube amplifier circuit comprising two stages of pentode vacuum tubescoupled with a feed back connection comprising a resistance and a capacitance connected in series between the plate of the second tube and the cathode of the first tube for the purpose of feedback a portion of the output of the second tube to the first tube, networks comprising interconnected resistances and capacitances connec't-' in the control grids with their corresponding cathodes. said networks providing a negative gridvcltaee on their respective control grids, said ne at ve grid voltage being of sufficient magnitude and so varyin with signal frequency as to hold the oi the amplifier circuit substantially constant. one o'f said networks comprising a .netwo k connected between the cathode of the secnd tube and the control grid of the second tube.

se id network comprising a resistance and capaciin parallel, one terminal of which is 0011- second tube and to a path to the common ground including another resistance.

LAURENCE E. ORNE. I

ground and the other terminal of whichis conl 

